<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="https://code.jquery.com/jquery-3.4.1.min.js"></script>
</head>

<body>
    <div>元素一</div>
    <div>元素二</div>
    <div>元素三</div>
    <script>
    var divs = document.getElementsByTagName('div');
    //转化jQuery对象
    var $divs = $(divs);
    var div_1 = $divs.first();
    div_1.css('color','red');
    </script>
</body>

</html>